Your code doesn't include any fetch statement. And as another answer notes, you need single quotes around $_SESSION["username"].

$result = mysql_query("select count(*) from registeredUsers where email='{$_SESSION['username']}'");
// Verify it worked
if (!$result) echo mysql_error();
$row = mysql_fetch_row($result);
// Should show you an integer result.
print_r($row);

Email codedump link for How to get the result of a select count(*) query in PHP?